Conventionally, secondary batteries such as a nickel metal hydride battery or the like have been used as a power source of, e.g., a rechargeable power tool set. The secondary batteries can be recharged within a shortened period of time by increasing a charging current. In this process, however, a great deal of heat is generated thereby raising the temperature of the batteries. This is problematic in that the secondary batteries may suffer from a shortened cycle life.
Thus, the conventionally known charging devices for charging such secondary batteries are provided with a cooling fan that cools down the secondary batteries to thereby restrain any temperature rise of the latter (see, e.g., Japanese Patent Laid-open Publication Nos. 2001-136676 and 2000-312440).
In the meantime, the secondary batteries have a tendency to show a reduced cycle life not only when they are charged at an elevated temperature but also when the charging task is carried out under a super-cooled condition beyond a suitable charging temperature range. The temperature range suitable for battery charging depends on the types of the secondary batteries. Moreover, the secondary batteries exhibit different behaviors depending on their types when the charging operation is performed. For example, a nickel metal hydride battery undergoes an exothermic reaction in which heat is generated along with the charging operation, whereas a lithium-ion or a nickel cadmium secondary battery is subject to an endothermic reaction in which heat is absorbed along with the charging operation.
For the very reason, in the charging device provided with a cooling fan for cooling down the afore-said secondary batteries, if the secondary batteries are cooled regardless of the types thereof, for instance, if the nickel metal hydride battery generating heat when on charge is cooled by the cooling fan in the same manner as applied to the lithium-ion secondary battery absorbing heat when charged, there may be such an instance that the secondary batteries are super-cooled to a temperature unsuitable for charging the same. This may lead to a problematic case that the cycle life of the secondary batteries is reduced by the charging operation.